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thermistor_troubleshooting [2016/04/16 17:12]
Traumflug [Thermistor Type]
thermistor_troubleshooting [2018/05/27 16:10]
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-====== Thermistor Troubleshooting ====== 
  
-Sometimes things simply don't work, despite everything looks good. In such cases it's the best idea to work step by step to find the cause. 
- 
-===== How It Should Work ===== 
- 
-{{ :​Troubleshooting:​Thermistor Circuitry.svg?​direct&​300|Typical thermistor circuitry.}} 
- 
-Resistors change their resistance with temperature. For fixed value resistors this effect is unwanted. Thermistors take advantage of this effect and do so in a predictable fashion. 
- 
-A typical thermistor circuitry is shown to the right. About all RepRap controllers use the same design. 
- 
-  * //CT1// means to smooth out noise, it's not directly involved in the measurement circuitry. 
-  * //​Connector//​ is where the thermistor is connected. 
-  * //RT1// makes up a [[https://​en.wikipedia.org/​wiki/​Voltage_divider|voltage divider]] together with the thermistor. Most controllers have a 4700\_ohms resistor here, which gives optimum resolution at about 100\_°C. 1000\_ohms moves the range of optimum resolution closer to typical extruder temperatures. 
- 
-Connecting components this way gives a voltage on the signal line which depends on the thermistor'​s resistance and accordingly,​ on the thermistor'​s temperature. If the thermistor is very hot, its resistance is near zero, so the signal'​s voltage will be close to zero. If the thermistor is very cold, resistance is very high, signal'​s voltage will beclose to the supplied voltage, 3.3\_volts (or 5\_volts on other controllers). In between is our measurement range. 
- 
-===== Hardware Measurements ===== 
- 
-Before searching for firmware misconfigurations,​ it's always a good idea to check hardware. A firmware can only report the voltage on the processor'​s signal pin. 
- 
-==== Thermistor Type ==== 
- 
-Actually, thermistor type doesn'​t matter much for troubleshooting. They all work by the same principle, so the circuitry shown above always works (unless it's faulty). Most RepRap printers use thermistors with 100\_kOhms nominal resistance. This means they have 100\_kOhms at 25\_°C ("room temperature"​). 
- 
-During firmware configuration types matter a lot more, because there we want not only a working principle, but also reasonably accurate readings. 
-==== Check Supply Voltage ==== 
- 
-Easiest way to do this is to disconnect the thermistor and measure voltage between both pins. With no thermistor connected, it should be full supply voltage, 3.3\_or 5\_volts depending on the controller. 
- 
-==== Check Thermistor Wiring ==== 
- 
-This measurement is simple, too. Disconnect the thermistor and measure resistance between both pins. At room temperature this should be around the nominal value of the thermistor, typically 10\_kOhms or 100\_kOhms. Warming the thermistor by hand should reduce this resistance. 
- 
-Measuring the thermistor while it's plugged in gives false results. 
- 
-If resistance is always zero, there'​s a short in your wiring. If resistance is infinite, wiring is broken. 
- 
-==== Check Voltage on the Processor Pin ==== 
- 
-{{ :​troubleshooting:​gen7_2.0_thermistor_measurements.jpeg?​direct&​300|Location of thermistor pins on a Gen7-ARM 2.0}} 
- 
-This is what the firmware actually "​sees",​ so it's crucial. Picture to the right shows where these pins are located on a Gen7-ARM 2.0. It's the 5th and 6th pin in the upper row. 
- 
-This measurement can be done without the processor installed. If one is installed, the firmware shouldn'​t make these pins an output, but keep the default, input. 
- 
-Without a thermistor connected, both pins should read full 3.3\_volts (5\_volts on 5\_V controllers). Connecting a thermistor at room temperature should drop this not much, but a bit. Typically to 3.2\_... 3.3\_volts. 
- 
-Warming the thermistor by hand might change that slightly, perhaps by another 0.05\_V drop. Warming the thermistor with a lighter (be careful to not overheat it!), should drop that voltage further, down to very low readings, like 0.5\_V. 
thermistor_troubleshooting.txt · Last modified: 2018/05/27 16:10 (external edit)